Investors and strategists have a lot of theories on what drove the latest surge in bond yields: Higher oil prices. Comments from Fed governor Michael Barr. Data showing strong economic growth yesterday. A weak Treasury auction.

Few are denying those were contributing factors. But there was also something else behind the scenes, according to market watchers: Traders being forced out of their positions.

"The move has the hallmarks of a pain trade and forced selling by investors at these more elevated levels and could have further to run," said MUFG Bank's Derek Halpenny in a note today.

Mohit Kumar, chief European economist at Jefferies, had the same assessment: "The main driver was likely stop outs and position unwinds. There appears to be a lot of pain on the street in fixed income."

In recent weeks, many traders had been making a popular fixed-income bet known as a steepener, betting that the gap between short- and long-dated bond yields would widen. Instead, that gap has narrowed as traders have rapidly adjusted their interest-rate expectations.

Active traders "accounts either had steepeners or outright longs at the front end of the curve. Some of the positions had been cleared in the last two weeks, but yesterday saw another round of washouts," Kumar said.